Current heat exchangers for use in automobiles in applications such as air conditioners are well known, and are generally of the flat plate type. These flat plate type heat exchangers, or evaporators as they are sometimes called, are constructed with alternating and adjacent laterally extending fluid flow and air flow passages. The refrigerant fluid passageways are provided with a plurality of fluid flow obstructions located therein and are formed by bonding together pairs of elongate plates having dimples located therein. The plurality of fluid flow obstructions so formed act to produce a tortuous flow path in the fluid flow passageways in order to produce turbulence and to increase the contact surface area between the walls of the passageway and the refrigerant fluid in order to increase the efficiency of heat transfer from the air to the fluid.
In one type of evaporator, the refrigerant fluid inlet and outlet ports are located adjacent the ends of the elongate plates, such as in U.S. Pat. Nos. 4,470,455 (Sacca) and 4,600,053 (Patel et al.). These ports are formed from raised portions, sometimes referred to as cups, located adjacent to the end portions of each plate. The raised portions are generally circular and have a lip portion in the bottom of the cup, the edge of which defines an aperture in the bottom of the cup. When the pairs of elongate plates are joined together, the cups in each plate of the pair are in registration and define either a fluid inlet or outlet passageway transversely therethrough. The fluid entering the inlet enters the lateral fluid passageways between the plates via entrances located in these opposed cup segments.
The evaporator is assembled by joining together a plurality of these joined pairs of plates. The plate pairs are coupled to each other around the lips at the bottoms of the cups and a solid seal is formed by brazing. In this way, a multi-plate assembly is built up. An air-flow passageway exists between adjacent joined pairs of plates in which a high surface area fin is located for efficient heat exchange.
In another type of evaporator, the inlet and outlet tanks containing the fluid ports are adjacent to each other and located at one end of the evaporator, such as disclosed in U.S. Pat. No. 4,696,342 (Yamauchi et al.) and U.S. Pat. No. 4,723,601 (Ohara et al.).
A drawback of these current evaporator designs is a loss of efficiency due to the fact that the full frontal area of the evaporators is not utilized since the refrigerant inlet and outlet tank portions containing the fluid passages are arranged along the full width of one or both sides thereof. Thus, the area taken up by the tank portions precludes the presence of fins, which results in a finned area/duct area ratio significantly less than unity and typically ranging from 0.70 to 0.80.
Another drawback of these evaporators using the above-mentioned drawn cup assembly is the necessity for tight and accurate control over the relative positioning of the two plates during assembly, since a good seal between the lip portions of adjacent cups is essential to proper functioning of the evaporator. Further to this, these types of high surface area and unsupported joints have low burst strengths and are prone to rupture. This will increasingly become a significant problem as current air conditioning refrigerants containing chlorine, e.g. R-12, are replaced by environmentally safer materials. Some of these, for example R-134, operate at higher vapour pressures than current refrigerants and therefore heat exchangers utilizing said alternative refrigerants will require greater burst strengths.
GB patent specification A-1,305,464 published Jan. 31, 1973 describes a sheet metal radiator assembly for the circulation of coolant oil from and to an electrical transformer. This heat exchanger assembly is made with a number of laterally spaced, upright plate units, each constituted in its entirety by a pair of thin sheet steel srampings. This assembly has top and bottom header portions which are formed by tubular extensions at the top and bottom of the plates, these tubular extensions telescoping into one another and being braised together. In this known construction, there are no fins arranged between the plate pairs and the spacing between the plate pairs is governed by shoulders formed about the base of the inner tubular connector used to form each header.
Previous prior art heat exchanger designs comprised long, small diameter tubes fed through a flat fin array wherein the tubes made multiple, parallel passes through the fin and therefore providing full frontal area air flow. A drawback to this design is the relatively low surface area which the hot fluid comes into contact with during flow through the heat exchanger due to the fluid being constrained to move through the tubes.
Still another drawback to certain prior are air conditioning evaporators relates to refrigerant fluid residence times in various parts of the evaporators. In has been observed that the refrigerant flow rate in certain portions of prior art evaporators is reduced over others, creating dead zones or spots, in other words, areas of low flow velocity such as large header tanks. Under operating conditions in the vicinity of the compressor exit ports, the refrigerant is susceptible to chemical breakdown thereby forming strong acids such as hydrochloric and hydrofluoric acid in the presence of trace water contaminant. These acids are known to cause corrosion and have produced pinhole leaks in these low flow zones.